The widespread use of copier/duplication machines in recent years has caused obvious problems associated with the ease and convenience of making copies of documents with these machines. For example, situations arise wherein it is desired that particular documents should not be reproduced or should be reproduced only by members of a defined control group. In particular, a document may contain information of a confidential or classified nature, the contents of which are meant to be restricted to a few individuals or only one individual. Further, the problem of unauthorized copying of copyrighted materials has not, at this date, been successfully addressed.
The prior art has generated various techniques which have sought to prevent unauthorized copying of documents, particularly in view of the proliferation of copying machines which makes it extremely easy to reproduce almost any document. One prior art technique relies on reducing the spectral contrast of the information on the selected document when it is being copied while maintaining the readability of the document without utilizing peripheral equipment. In particular, U.S. Pat. No. 3,713,861 discloses a technique wherein light emanating from the exposure source is directed to the surface of a document, the surface having been overcoated with a selected fluorescent film, the entire document fluorescing when irradiated by the light thereby eliminating contrast (and therefore no image impression) on the copy. The disadvantage to this technique is that a fluorescent coating must be selected and applied to each document to be made copy-proof, the attendant handling procedures causing obvious problems. Further, the fluorescent coating must be selected to absorb most of the radiation emitted from the exposure light source. In this case, if a document is coated with a fluorescent material which absorbs radiation in the region between 5100-5350 and the light emitted by the exposure light source is in the region between 5100-5500A., the disclosed system would be effective. However, if the copying device utilizes an exposure source emitting radiation in a region greater than 6000A., the system would be ineffective.
Another prior art technique prints specularly reflective patterns or spectral background on a document with a particularly colored ink (i.e., blue or yellow). However, this technique is only effective with copying machines having a limited spectral response.
In general, however, as the spectral response of copying machines improves, it becomes increasingly difficult to produce non-copyable documents which can be easily read without peripheral equipment.
Another solution to the problems suggested by the aforementioned patent is to make the master copy on an outsized paper, thereby making it difficult to reproduce master copies because they would not conform easily to the exposure station of conventional copying machines. The solution is impractical from a handling and expense point of view since special paper will have to be cut and images enlarged or reduced for placement on the copier, and copying is not prevented but only made inconvenient.
A proposed technique to solve the aforementioned problem would be to modify existing copying machines whereby an event would be triggered when a sensitive document is to be copied. This sytem requires that the original documents have a code, or signature, which the modified copy machine can recognize. Typical codes include either visible or invisible codes, patterns, or marks written with materials that can be identified by any of the following characteristics: ultraviolet or infrared radiation, visual reflective or transmissive, magnetic, electrostatic or chemical. For example, a chemical additive could be added to the sensitive document paper which could be chemically "sniffed" by apparatus in the copying machine. A drawback in the use of a code recognition copying system as proposed in the prior art would be that the system would be complex and expensive to make.
Therefore, copying machine inexpensively modified to optically respond to codes on a sensitive document would be desirable. In particular, a technique which optically correlates an image from an optically patterned document with a comparison image, and detects a correlation signal which is used to inhibit and/or control copying of a non-conforming document and would be simply incorporated in existing copying machines would be an advance in the prior art.